On the Gantt Chart Options menu set the Time Units -> Minor Units to Hours and the Duration Units Format to Hours.
If the rest of the chart is in weeks or months then it might be best to have a separate chart for these tasks. You can't mix time units in one chart.
P.S. If you have a bunch of "that's just a five minute job" tasks, you can guarantee that you can only get a maximum of three of these completed in one working day!